The knowledge of aerosols particle size and concentration is highly relevant for many applications dealing with aerosols such as the dissemination of insecticides and the emission of stacks. In the defense and security area, the standoff detection of bioaerosol clouds with spectrometric LIDAR is now a reality (Simard et al., 2004, IEEE Trans. On Geoscience and Remote Sensing 42: 865-874). This technique has the advantages of rapidly monitoring the atmosphere over a wide area without physical intrusions and reporting an approaching threat before it reaches sensitive sites. However, the lack of information on bioaerosols particle size does not permit accurate measurement of the concentration. The knowledge of the size could also be used to reduce the rate of false alarm since pollens are significantly larger than bioaerosols agents. With the Multiple-Field-Of-View (MFOV) lidar developed at DRDC Valcartier, we have already demonstrated that the multiple scattering of a laser beam propagating in the atmosphere contains information on the size of the aerosols (Lidar: Range-Resolved Optical Remote Sensing of the Atmosphere, chapter 3: Lidar and Multiple Scattering, Editor Claus Weikamp (Springer Series in Optical Sciences), 2005, 455 pp, ISBN 0-387-40075; N. Roy et al., 2004, “Measurement of the azimuthal dependence of cross-polarized lidar returns and its relation to optical depth,” Appl. Opt. 43, 2777-2785).
These innovative results and recent developments with gated-ICCD (Intensified Charged Coupled Device) cameras have led us to perform a preliminary investigation on the determination of background aerosols and bioaerosols size and concentration. The difficulty with bioaerosol clouds is that they are sub-visible and therefore their low concentrations do not produce strong multiple scattering signals. In addition, they are mixed with background aerosols. In this paper, we demonstrate that it is possible to obtain size information on low concentration of bioaerosols using the MFOV lidar principle and the concept of background aerosol subtraction. We present the theory of the concept and demonstrate experimentally its validity on effective diameter retrieval of Elm and Timothy low concentration pollens.